// f: (int w, int h) -> BufferedImage
static ImageSurface jcanvas(O f) {
  ret jcanvas(f, 500);
}

static ImageSurface jcanvas(O f, final int updateDelay) {
  assertAWTThread();
  final new ImageSurface is;
  onResize(is, r {
    awtLater(is, updateDelay, r {
     BufferedImage img = is.getImage();
      int w = is.getWidth(), h = is.getHeight();
      if (img.getWidth() != w || img.getHeight() != h)
        is.setImage(asBufferedImage(callF(f, w, h)));
    });
  });
  ret is;
}